The present invention relates generally to the drilling of oil and gas wells, or similar drilling operations, and in particular to orientation of tooth angles on a roller cone drill bit.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 5. In conventional vertical drilling, a drill bit 50 is mounted on the end of a drill string 52 (drill pipe plus drill collars), which may be miles long, while at the surface a rotary drive (not shown) turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, the fixed or drag bit, seen in FIG. 4, and the roller cone bit, seen in FIG. 3. In the roller cone bit a set of cones 36 (two are visible in this drawing) having teeth or cutting inserts 38 are arranged on rugged bearings on the arms 37 of the bit. As the drill string is rotated, the cones will roll on the bottom of the hole, and the teeth or cutting inserts will crush the formation beneath them. Drilling fluid, which is pumped down the drill string under pressure, is directed out nozzles 34, to provide cleaning of the bit and to sweep broken fragments of rock uphole.
Background: Roller Cone Bit Design
FIG. 2 is view from the bottom of a roller cone bit which has three cones 201, 202, and 203, each containing rows of chisel-shaped inserts 210 for cutting elements. It can be noted that the xe2x80x9cconesxe2x80x9d in a roller cone bit need not be perfectly conical (nor perfectly frustroconical), but often have a slightly swollen axial profile. Moreover, the axes of the cones do not have to intersect the centerline of the borehole, as can be seen in this drawing. (The angular difference is referred to as the xe2x80x9coffsetxe2x80x9d angle.) Another variable is the angle by which the centerline of the bearings intersects the horizontal plane of the bottom of the hole, and this angle is known as the journal angle. Thus as the drill bit is rotated, the cones typically do not roll true, and a certain amount of gouging and scraping takes place. The gouging and scraping action is complex in nature, and varies in magnitude and direction depending on a number of variables.
It should also be noted that while each cone has a row of teeth circumscribing its greatest circumference (this is the heel, or gage, row), the other rows of teeth are offset so that no two cones have teeth which will intersect each other as they rotate.
Conventional roller cone bits can be divided into two broad categories: Insert bits and steel-tooth bits. Steel tooth bits are utilized most frequently in softer formation drilling, whereas insert bits are utilized most frequently in medium and hard formation drilling.
Steel-tooth bits have steel teeth formed integral to the cone. (A hard-facing is typically applied to the surface of the teeth to improve the wear resistance of the structure.) Insert bits have very hard inserts (e.g. specially selected grades of tungsten carbide) press-fitted into holes drilled into the cone surfaces. The inserts extend outwardly beyond the surface of the cones to form the xe2x80x9cteethxe2x80x9d that comprise the cutting structures of the drill bit.
The design of the component elements in a rock bit are interrelated (together with the size limitations imposed by the overall diameter of the bit), and some of the design parameters are driven by the intended use of the product. For example, cone angle and offset can be modified to increase or decrease the amount of bottom hole scraping. Many other design parameters are limited in that an increase in one parameter may necessarily result in a decrease of another. For example, increases in tooth length may cause interference with the adjacent cones.
Background: Tooth Design
The teeth of steel tooth bits are predominantly of the inverted xe2x80x9cVxe2x80x9d shape. The included angle (i.e. the sharpness of the tip) and the length of the tooth will vary with the design of the bit. In bits designed for harder formations the teeth will be shorter and the included angle will be greater. Gage row teeth (i.e. the teeth in the outermost row of the cone, next to the outer diameter of the borehole) may have a xe2x80x9cTxe2x80x9d shaped crest for additional wear resistance.
The most common shapes of inserts are spherical, conical, and chisel. Spherical inserts have a very small protrusion and are used for drilling the hardest formations. Conical inserts have a greater protrusion and a natural resistance to breakage, and are often used for drilling medium hard formations.
Chisel shaped inserts have opposing flats and a broad elongated crest, resembling the teeth of a steel tooth bit. Chisel shaped inserts are used for drilling soft to medium formations. The elongated crest of the chisel insert is normally oriented in alignment with the axis of cone rotation, as can be seen in FIG. 2. Thus, unlike spherical and conical inserts, the chisel insert may be directionally oriented about its center axis. (This is true of any tooth which is not axially symmetric.) The angle of orientation is measured as a deviation from the plane intersecting the center of the cone and the center of the tooth.
Background: Roller Cone Tracking
The study of bottom hole patterns has allowed engineers to evaluate performance and to begin to reduce such phenomena as tracking. FIG. 6A shows a computer generated pattern of the impressions of the teeth of a single roller cone on the hole bottom after a single revolution of the bit, showing a large separation between the individual teeth impressions, and between the rows on the cone.
FIG. 6C shows the impression of all of the cones on the bit after a single revolution of the bit. Note that while the inner rows of teeth from different cones do not generally follow the same path as they traverse the hole bottom, the teeth in the heel row of all of the cones tend to follow a single path on the outer circumference of the hole.
Tracking occurs when the teeth of a drill bit fall into the impressions in the formation formed by other teeth at a preceding moment in time during the revolution of the drill bit. FIG. 6B shows an impression of a single cone on the hole bottom after two revolutions of the bit. In this case, many of the impressions from the first revolution are partially overlain by the impressions of that same row from the second revolution. This overlapping will put lateral pressure on the teeth, tending to cause the cone to align with the previous impressions. Tracking can also happen when teeth of one cone""s heel row fall into the impressions made by the teeth of another cone""s heel row. Tracking results in slow rates of penetration, detrimental wear of the cutting structures and premature failure of bits.
Background: Bit Design to Prevent Tracking
The economics of drilling a well are strongly reliant on rate of penetration, which is itself strongly affected by the design of the cutting structures. Currently, roller cone bit designs remain the result of generations of modifications made to original designs. The modifications are based on years of experience in evaluating bit records, dull bit conditions, and bottom hole patterns, but these bit designs have not solved the issue of tracking.
One method commonly used to discourage bit tracking is known as a staggered tooth design. In this design the teeth are located at unequal intervals along the circumference of the cone. This is intended to interrupt the recurrent pattern of impressions on the bottom of the hole. However, staggered tooth designs do not prevent tracking of the outermost rows of teeth, where the teeth are encountering impressions in the formation left by teeth on other cones. Staggered tooth designs also have the short-coming that they can cause fluctuations in cone rotational speed and increased bit vibration.
U.S. Pat. No. 5,197,555 to Estes discloses milled-tooth cones with xe2x80x9cthe gage [row] of one cone oblique to the leading side and the gage row of another cone oblique to the trailing sidexe2x80x9d.
Roller Cone Bits, Methods, and Systems with Anti-Tracking Variation in Tooth Orientation
The present application discloses new bit and cone designs, as well as methods of design and systems and drilling methods using these designs, in which variation in tooth orientation is used to reduce tracking. (Of course, tooth orientation is only relevant if the teeth are not axisymmetric, e.g. with chisel shaped insert teeth.) At least two classes of embodiments are disclosed, which can be used separately or (to achieve a synergistic result) together.
The parent application described bit design procedures using control of tooth orientation as one of the design variables. In implementing those procedures, the present inventor realized that the variation in tooth orientation which is described in that application can also achieve a substantial improvement in tracking resistance. When one tooth""s intrusion into the formation partly overlaps the impression made by a preceding tooth, a lateral force will result which tends to align the intrusion with the impression. However, the present inventor has realized that, when a tooth""s orientation does NOT allow it to fully fit into the impression made by a previous tooth, the lateral force tending to pull the tooth toward the impression will be reduced (though typically not eliminated). By varying tooth orientation to avoid perfect fit between an impression and a following tooth (in some cases), the propensity to track can be reduced. The less perfect the match between one tooth and another, the more the propensity to track is reduced; for example, with chisel-shaped teeth, the maximum reduction in lateral force is achieved if a tooth is 90 degrees out of alignment with a following tooth; but significant reductions can be achieved even with 30 degrees of misalignment.
Co-pending application Ser. No. 09/387,304, filed Aug. 31, 1999, now U.S. Pat. No. 6,095,262 and which is hereby incorporated by reference, discloses a method of optimizing the tooth orientation on a cone. It is herein disclosed that within an optimal range of orientations, the tooth orientation within a single row or between the heel rows of two or more cones can be varied to lessen the propensity for tracking. It is understood in this context that references to xe2x80x9ctoothxe2x80x9d or xe2x80x9cteethxe2x80x9d include both milled teeth and elongated inserts, and that the invention is not specifically limited to the use of steel teeth.
In one class of embodiments, the orientations of the teeth are varied between the heel row of one cone and the heel row of another cone. Since the heel rows of all three cones normally follow the same path, reduction in tracking propensity is particularly useful here.
In another class of embodiments, the orientations of the teeth are varied within a single row of a cone. This helps to avoid same-row tracking forces: tracking is not only caused by the impressions of a preceding cone. The inner rows of teeth are usually spaced so that no two rows follow the same path on the cutting face; but a single row of teeth, on a single cone, will still encounter the impressions left by its own previous path. Since a full circle of a row""s path will not necessarily be an exact multiple of the spacing of impressions on the cutting face, the misalignment of teeth to previous impressions may indeed contribute a lateral force component. Here too a difference in orientation between tooth and impression helps to reduce this lateral force component. The different tooth orientations can be grouped in blocks in a given row, such as a block of teeth with orientation A which extends over half the row circumference and a block of teeth with orientation B which extends over the other half; or blocks ABAB, where each block extends over 90 degrees; or blocks ABC; or the blocks can have unequal numbers of teeth.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
reduces propensity to track rows on different cones that drill the same circumferential path of the hole bottom;
reduces propensity to track the other teeth on the same row of a cone;
minimizes vibration during drilling;
increases lifetime of drill bit and drill string components;
reduces drilling cost-per-foot.